Gas turbine engine rotors are typically configured with removable blading in fan, compressor and turbine stages to facilitate cost effective replacement of damaged or worn out components. For example, fan and compressor blading may be damaged by foreign objects such as pebbles, birds and ice which may be ingested into the engine. Alternatively, turbine blading is conventionally life limited, requiring replacement after a predetermined number of flight hours or engine cycles, due to the harsh, high temperature corrosive environment in which the blading operates.
According to one attachment scheme, a plurality of blades have contoured dovetails disposed in a common number of complementarily shaped dovetail slots formed in a rim portion of a disk. During engine operation, the cantilevered blading is subject to complex, multi-order modes of vibration due to interaction of the airfoils thereof with primary engine airflow. Depending on the mode and amplitude of vibration, high cycle fatigue cracks may be initiated and propagated in high stress regions of the blade, for example in the airfoil, dovetail or shank.
In order to limit the detrimental effects of such vibration, especially at blade resonant frequencies which fall within the operating range of the engine, damping schemes are routinely employed which convert the vibratory energy, using the motion associated therewith, into dissipated heat. There exist two basic damping schemes: blade-to-ground damping and blade-to-blade damping. One such blade-to-ground damper apparatus is disclosed in U.S. Pat. No. 5,226,784 entitled "Blade Damper" granted to Mueller et al on Jul. 13, 1993 and assigned to the same assignee as the present invention. According to Mueller et al, a damper of triangular cross section disposed in a chamber in a disk post contacts a radial face of the chamber along one leg and a blade platform along an hypotenuse thereof. Vibratory induced motion of the blade platform produces advantageous slippage at the abutting surfaces.
Blade-to-blade damper apparatus are disclosed in U.S. Pat. No. 5,156,528 entitled "Vibration Damping of Gas Turbine Engine Buckets" granted to Bobo on Oct. 20 1992 and U.S. Pat. No. 5,302,085 entitled "Turbine Blade Damper" granted to Dietz et al on Apr. 12, 1994, both of which are also assigned to the same assignee as the preset invention. While relying on the same friction mechanism for damping as the aforementioned blade-to-ground damper, blade-to-blade dampers are fundamentally different in that these dampers are disposed between portions of adjacent blades, for example, proximate blade platform edges. Depending on the mode and amplitude of vibratory induced motion sought to be attenuated, one damping scheme may be more beneficial than the other. For example, while blade-to-blade dampers are relatively ineffective at damping in-phase motion of adjacent blades, such dampers may be highly effective when blade motion is out of phase, as discussed by Mueller et al.
Blade-to-blade dampers may also be advantageously employed to seal the gap formed between adjacent platform edges to prevent the migration of hot flowpath gases into the rim region of the disk, especially in a turbine stage. Effective damping and sealing however necessitate a stable damper configuration. The centrifugally loaded damper and mating platform surfaces must cooperate, providing sliding contact therebetween. If the radial damper load is too high or the included angle too acute, the damper may lock to one or both mating platform surfaces. In this case, while acting as an effective seal, the damper provides little or no damping as relative movement is prevented.
An effective damper apparatus should provide substantially continuous sliding contact between the damper and associated blade structures for all operating conditions and tolerance stacks. Ensuring such a condition has been problematic. Due to the inherent variability associated with both the orientation and movement of abutting surfaces, maintenance of sliding contact between the damper and both blades simultaneously is troublesome. Conventional triangular shaped dampers often exhibit bistable rocking, sustaining planar contact first with one blade platform then the other, depending on the movement of the platforms. Contributing to this instability are a variety of factors such as initial damper orientation and vertex angle, mating platform surface location and orientation as well as component tolerances associated therewith. Damper instability results in both intermittent damping and ineffective sealing.